Rubber composition and method for producing same

ABSTRACT

A rubber composition obtained by dry-mixing a natural rubber wet master batch yielded by mixing at least a natural rubber latex and a carbon-black-containing slurry solution with each other in a liquid phase and drying the resultant mixture, a dry rubber made mainly of a polybutadiene rubber, and an oil, wherein when the total amount of rubber components in the rubber composition is regarded as 100 parts by mass, the natural rubber is contained in an amount of 50 parts or more by mass, and the polybutadiene rubber is contained in an amount of 20 to 50 parts by mass, and the oil has a pour point of −10 C or lower, and an aniline point of 90 C or higher, and the blend amount of the oil is from 15 to 40 parts by mass for 100 parts by mass of the rubber components.

This Application is a Divisional of U.S. application Ser. No. 14/372,932filed Jul. 17, 2014, which is a 371 National Stage of PCT/JP2012/081683filed Dec. 6, 2012, which claims priority to Japanese Patent ApplicationNo. 2012-067543 filed Mar. 23, 2012, all of which are hereinincorporated by reference.

TECHNICAL FIELD

The present invention relates to a rubber composition, and a method forproducing the same, specifically, a rubber composition that can belargely decreased in E′ storage elastic modulus in a low temperaturerange, can be greatly improved, particularly, in braking performance onan ice and snow road surface (referred to also as an “ice road surface”hereinafter) when used as a raw material for tires, for example, theirtreads, and can be further improved in low-thermogenic performance.

BACKGROUND ART

Rubbers for treads of studless tires used for running on ice roadsurfaces are required to have excellent on-ice and on-snow performances.In order to improve tires in frictional performances on ice roadsurfaces, for example, the following method has been hitherto adopted: amethod of not only using a polybutadiene rubber, which is low in glasstransition temperature, or blending a softening agent, such as oil, intoa tread rubber to maintain a low hardness of the tread rubber even atlow temperatures, thereby improving the rubber in hysteresis frictionalperformance, but also blending a hard material such as hollow particles,glass fiber or plant granular material thereinto, thereby improving therubber in scratch frictional performance. In order to improve the treadrubber, particularly, in gripping performance on a wetting road surface(referred to also as a “wet road surface” hereinafter), a method isadopted in which the blend proportion of a filler or a softening agentsuch as oil is increased in the composition of the rubber. However, avulcanized rubber obtained therefrom tends to be lowered inlow-thermogenic performance and abrasion resistance.

Apart from the above, in the rubber industry, known is the use of anatural rubber wet master batch for improving a rubber compositioncontaining a filler, such as carbon black, in workability when thiscomposition is produced, or in filler dispersibility therein (forexample, Patent Document 1 listed below). This is a technique of:mixing, in a liquid phase, a natural rubber latex with afiller-containing slurry solution obtained by mixing a filler and adispersing solvent with each other at a predetermined ratio beforehandand then dispersing the filler into the dispersing solvent by mechanicalforce; subsequently adding a solidifier such as an acid to the mixtureto solidify the mixture; collecting the solidified product; and thendrying the product. The use of the natural rubber wet master batch givesa rubber composition better in filler dispersibility therein and inrubber physical properties such as workability and reinforceability ascompared with the use of a rubber dry master batch obtained bydry-mixing a filler with a rubber.

Patent Document 2 listed below describes a rubber composition forice-and-snow-road tire treads which contains: a natural rubber wetmaster batch containing a carbon black having a nitrogen adsorptionspecific surface area of 105 to 155 m²/g, and a CTAB adsorption specificsurface area of 100 to 150 m²/g; and a polybutadiene rubber.

Furthermore, Patent Document 3 listed below states that a rubber wetmaster batch which is one wherein a filler is evenly dispersed and thefiller is restrained from being re-aggregated with time, and which isusable as a raw material for a vulcanized rubber excellent inlow-thermogenic performance, endurance and rubber strength can beproduced by a rubber-wet-master-batch-producing method including step(I) of adding, when the filler is dispersed into a dispersing solvent,at least one portion of a rubber latex solution thereto, therebyproducing a slurry solution containing the filler to which rubber latexparticles adhere, step (II) of mixing this slurry solution with the restof the rubber latex solution to produce a rubber latex solutioncontaining the just-above described rubber-latex-particle-adheringfiller, and step (III) of solidifying and drying the rubber latexsolution containing the rubber-latex-particle-adhering filler.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-2004-99625

Patent Document 2: JP-A-2004-107482

Patent Document 3: Japanese Patent No. 4738551

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

However, the inventor has made eager investigations so that a vulcanizedrubber from the rubber composition described in each of Patent Documents1 and 2 has turned out to be raised in E′ in a low temperature range soas to be deteriorated in braking performance on an ice road surface, andbe also deteriorated in low-thermogenic performance.

The rubber-wet-master-batch-containing rubber composition described inPatent Document 3 is excellent in filler dispersibility therein, and isimproved in low-thermogenic performance. However, it has become evidentthat when this rubber composition is used for, for example, a tiretread, there remains room for a further improvement in brakingperformance, particularly, on ice road surfaces.

In light of this actual situation, the present invention has been made.An object thereof is to provide a rubber composition that can be largelydecreased in E′ in a low temperature range, can be greatly improved,particularly, in braking performance on an ice and snow road surfacewhen used as a raw material for tires, for example, their treads, andcan be further improved in low-thermogenic performance; and a method forproducing the composition.

Means for Solving the Problems

The object can be attained by the present invention as describedhereinafter. Accordingly, the present invention relates to a rubbercomposition obtained by dry-mixing a natural rubber wet master batchyielded by mixing at least a natural rubber latex and acarbon-black-containing slurry solution with each other in a liquidphase and drying the resultant mixture, a dry rubber made mainly of apolybutadiene rubber, and an oil, wherein when the total amount ofrubber components in the rubber composition is regarded as 100 parts bymass, the natural rubber is contained in an amount of 50 parts or moreby mass, and the polybutadiene rubber is contained in an amount of 20 to50 parts by mass, and the oil has a pour point of −10° C. or lower, andan aniline point of 90° C. or higher, and the blend amount of the oil isfrom 15 to 40 parts by mass for 100 parts by mass of the rubbercomponents.

Since the rubber composition according to the present invention is acomposition obtained by dry-mixing the natural rubber wet master batchwith the dry rubber, which is made mainly of the polybutadiene rubber,at the specific ratio, the composition is excellent in carbon blackdispersibility therein and can be decreased in E′ in a low temperaturerange. Furthermore, the specific amount of the oil having specificphysical properties (the pour point and the aniline point) is blendedinto the composition; thus, the composition can keep the rubber hardnessthereof at a low level even in a low temperature range to be improved inbraking performance on ice road surfaces. When the natural rubber wetmaster batch is, particularly, a master batch produced through thefollowing steps, the rubber composition is especially excellent infiller dispersibility therein: step (I) in which when a carbon black isdispersed into a dispersing solvent to prepare thecarbon-black-containing slurry solution, at least one portion of thenatural rubber latex is added thereto, thereby producing a slurrysolution containing the carbon black to which natural rubber latexparticles adhere, step (II) of mixing this slurry solution with the restof the natural rubber latex to produce a natural rubber latex solutioncontaining the just-above describednatural-rubber-latex-particle-adhering carbon black, and step (III) ofsolidifying and drying the natural rubber latex solution containing thenatural-rubber-latex-particle-adhering carbon black. Consequently, whenthese rubber components are used together with the specific oil, thecomposition is improved, with an especially good balance, in brakingperformance on ice road surfaces and low-thermogenic performance.

As described above, a vulcanized rubber from the rubber compositionaccording to the present invention is excellent in low-thermogenicperformance, and further in braking performance on ice road surfaces.Accordingly, the rubber composition according to the present inventionis particularly useful as a rubber composition for studless tires.

In the rubber composition, it is preferred that the natural rubber wetmaster batch contains the carbon black in an amount of 40 to 70 parts bymass for 100 parts by mass of the rubber components of the master batch.It is more preferred that the carbon black contained in the naturalrubber wet master batch has a nitrogen adsorption specific surface area(N₂SA) of 100 m²/g or less. According to this form, the rubbercomposition is further improved in low-thermogenic performance and cankeep the viscosity thereof at a low level to be also excellent inworkability.

When the total amount of the present rubber components in the rubbercomposition is regarded as 100 parts by mass, further when the totalamount of the present rubber components in the rubber composition isregarded as 100 parts by mass, at least one of a plant granularmaterial, a grain granular material, and a granular region of a graincore material may be further contained in an amount of 0.5 to 10 partsby mass. In this case, the vulcanized rubber is improved in scratchfrictional performance to be further improved in braking performance onice road surfaces.

Additionally, the present invention relates to a method for producing arubber composition comprising a natural rubber wet master batch obtainedby use of a carbon black, a dispersing solvent and a natural rubberlatex as raw materials, a dry rubber made mainly of a polybutadienerubber, and an oil, including: step (I) in which when the carbon blackis dispersed into the dispersing solvent, at least one portion of thenatural rubber latex is added thereto, thereby producing a slurrysolution containing the carbon black to which natural rubber latexparticles adhere, step (II) of mixing this slurry solution with the restof the natural rubber latex solution to produce a natural rubber latexsolution containing the just-above describednatural-rubber-latex-particle-adhering carbon black, step (III) ofsolidifying the rubber latex solution containing thenatural-rubber-latex-particle-adhering carbon black to produce thecarbon-black-containing natural rubber solidified product, and thendrying the solidified product to produce a natural rubber wet masterbatch, and step (IV) of dry-mixing this natural rubber wet master batchwith the dry rubber, which is made mainly of the polybutadiene rubber,and the oil, wherein when the total amount of rubber components in therubber composition is regarded as 100 parts by mass, the natural rubberis contained in an amount of 50 parts or more by mass, and thepolybutadiene rubber is contained in an amount of 20 to 50 parts bymass, and the oil has a pour point of −10° C. or lower, and an anilinepoint of 90° C. or higher, and the blend amount of the oil is from 15 to40 parts by mass for 100 parts by mass of the rubber components. Thisproducing method makes it possible to produce a rubber compositionimproved, with an especially good balance, in braking performance on iceroad surfaces, and low-thermogenic performance.

In this producing method, it is preferred that: the step (III) has atleast a dehydrating step (III-A) of using a first uniaxial extruder todehydrate the carbon-black-containing natural rubber solidified productwhile the product is heated up to 100 to 180° C., thereby producing anatural rubber wet master batch, and a drying plasticizing step (III-B)of using a second uniaxial extruder to plasticize the natural rubber wetmaster batch while the master batch is heated up to 120 to 180° C.,thereby producing the just-above described natural rubber wet masterbatch further decreased in water content by percentage; and this rubbercomposition producing method has no cooling step between the dehydratingstep (III-A) and the drying plasticizing step (III-B).

According to this producing method, the first uniaxial extruder is usedto dehydrate the carbon-black-containing natural rubber solidifiedproduct while the product is heated up to 100 to 180° C., therebyproducing a natural rubber wet master batch (the dehydrating step(III-A)); thus, the water content by percentage in the resultant naturalrubber wet master batch can be efficiently decreased while the appliedheat capacity and mechanical energy are restrained as much as possible.Furthermore, the second uniaxial extruder is used to plasticize thisnatural rubber wet master batch while the master batch is heated up to120 to 180° C. (the drying plasticizing step (III-B)), thereby making itpossible to produce the natural rubber wet master batch which is furtherdecreased in water content by percentage and is hardlyrubber-deteriorated. As a result, a rubber composition can be finallyobtained which is improved in rubber physical properties, such astearing resistance and stress property in its high-strain region.

For example, if the natural rubber wet master batch after thedehydrating step (III-A) is cooled down to room temperature not laterthan the drying plasticizing step (III-B), a larger heat capacity andmechanical energy would be given to the natural rubber wet master batchwhen the drying plasticizing step (III-B) is performed. However, thepresent producing method has no cooling step between the dehydratingstep (III-A) and the drying plasticizing step (III-B); thus, the watercontent by percentage in the resultant natural rubber wet master batchcan be efficiently decreased while the heat capacity and mechanicalenergy applied to the master batch are restrained as much as possible. Amanner for preventing the natural rubber wet master batch from beingcooled between the dehydrating step (III-A) and the drying plasticizingstep (III-B) may be, for example, a manner of connecting the firstuniaxial extruder and the second uniaxial extruder to each other. The“cooling step” referred to in the present invention denotes, forexample, a step in which the rubber wet master batch after thedehydrating step is cooled to a temperature of 40° C. or lower; anddenotes, in a broader sense, a step in which the master batch is cooledto a temperature of 60° C. or lower.

In the present rubber composition producing method, the water content bypercentage of the natural rubber wet master batch obtained through thedehydrating step (III-A) is preferably from 1 to 10%. The water contentby percentage of the natural rubber wet master batch obtained throughthe drying plasticizing step (III-B) is preferably 0.9% or less. Inacase where in the present invention a drying process extending over thetwo stages (the dehydrating step (III-A) and the drying plasticizingstep (III-B)) is performed, and further the water content by percentageafter each of the steps is set in the range described just above, therubber wet master batch can be efficiently decreased in water content bypercentage while a vulcanized rubber to be finally obtained therefrom iscertainly prevented from being rubber-deteriorated.

In the rubber composition producing method, it is preferred that in thedrying plasticizing step (III-B), a mechanical energy of 70 W/kg or lessis applied to the rubber wet master batch inside the second uniaxialextruder. In this case, the final vulcanized rubber can be preventedfrom being rubber-deteriorated with a higher certainty while the watercontent by percentage is efficiently decreased.

In the rubber composition producing method, it is preferred that whenthe natural rubber wet master batch is plasticized in the dryingplasticizing step (III-B), an anti-aging agent is added and blendedthereinto. In this case, the anti-aging agent can be more evenlydispersed in the natural rubber wet master batch, so that the finalvulcanized rubber can be prevented from being rubber-deteriorated withan even higher certainty. In the natural rubber wet master batch thathas undergone the dehydrating step (III-A), the water content bypercentage is decreased; thus, at the time of the drying plasticizingstep (III-B), it hardly happens that the anti-aging agent flows out withthe volatilization of water.

In the rubber composition producing method, it is preferred that thestep (III) includes, after the drying plasticizing step (III-B), ashaping plasticizing step (III-C) of using a mixer further to plasticizethe natural rubber wet master batch. In the shaping plasticizing step(III-C), it is more preferred that a mechanical energy of 70 W/kg orless is applied to the rubber wet master batch inside the mixer. In thiscase, the water content by percentage can be more efficiently decreasedwhile the final vulcanized rubber is certainly prevented from beingrubber-deteriorated. The mixer may be, for example, an open roll or auniaxial extruder.

Mode for Carrying Out the Invention

About the rubber composition according to the present invention, as araw material thereof, a natural rubber wet master batch is used which isyielded by mixing at least a natural rubber latex and acarbon-black-containing slurry solution with each other in a liquidphase and drying the resultant mixture. It is preferred to use,particularly, a natural rubber wet master batch produced through thefollowing steps since the rubber composition is remarkably good incarbon black dispersibility therein and a vulcanized rubber therefrom isimproved in low-thermogenic performance: step (I) in which when a carbonblack is dispersed into a dispersing solvent to prepare thecarbon-black-containing slurry solution, at least one portion of thenatural rubber latex is added thereto, thereby producing a slurrysolution containing the carbon black to which natural rubber latexparticles adhere, step (II) of mixing this slurry solution with the restof the natural rubber latex to produce a natural rubber latex solutioncontaining the just-above describednatural-rubber-latex-particle-adhering carbon black, and step (III) ofsolidifying and drying the natural rubber latex solution containing thenatural-rubber-latex-particle-adhering carbon black.

The natural rubber latex solution is a natural product produced by ametabolism effect of a plant, and is in particular preferably a naturalrubber/water system solution, in which a dispersing solvent therein iswater. The number-average molecular weight of a natural rubber in thenatural rubber latex used in the present invention is preferably2,000,000 or more, more preferably 2,500,000 or more. About the naturalrubber latex, a concentrated latex, and a fresh latex called a fieldlatex are usable without any discrimination. When the total amount ofthe rubber components in the rubber composition is regarded as 100 partsby mass, it is preferred that the natural rubber (solid) is contained inan amount of 50 parts or more by mass.

The carbon black may be any carbon black species usable in an ordinaryrubber industry, such as SAF, ISAF, HAF, FEF or GPF, or may be anyelectroconductive carbon black species such as acetylene black orKetjenblack. The carbon black may be a granulated carbon black species,which has been granulated, considering the handleability thereof in anordinary rubber industry, or a non-granulated carbon black species.

In the case of using, as the carbon black, one having a nitrogenadsorption specific surface area (N₂SA) of 100 m²/g or less, theresultant vulcanized rubber is more remarkably good in low-thermogenicperformance, and further the rubber composition can keep the viscositythereof at a low level to be also remarkably good in workability, whichis preferred.

The dispersing solvent is in particular preferably water, and may be,for example, water containing an organic solvent.

Hereinafter, a description will be made about the natural rubber wetmaster batch produced through the steps (I) to (III).

(1) Step (I)

The step (I) is a step of adding, when a carbon black is dispersed intoa dispersing solvent, at least one portion of a natural rubber latexthereto, thereby producing a slurry solution containing the carbon blackto which natural rubber latex particles adhere. It is allowable to mixthe natural rubber latex beforehand with the dispersing solvent, andthen add the carbon black thereto to disperse the carbon black therein.It is also allowable to add the carbon black into the dispersingsolvent, and next disperse the carbon black in the dispersing solventwhile the natural rubber latex is added thereto at a predeterminedadding speed; or add the carbon black into the dispersing solvent, andnext disperse the carbon black in the dispersing solvent while multipledivided fractions of the natural rubber latex are added thereto, thesefractions being equal to each other in volume. By dispersing the carbonblack into the dispersing solvent in the state that the natural rubberlatex is present, the above-mentioned slurry solution can be produced,which contains the natural-rubber-latex-particle-adhering carbon black.In the step (I), the addition amount of the natural rubber latex is, forexample, from 0.075 to 12% by mass of the whole of the used naturalrubber latex (the total of the amounts added in the steps (I) and (II)).

In the step (I), in the added natural rubber latex, the percentage bymass of the solid (rubber) to the carbon black therein is preferablyfrom 0.25 to 15%, more preferably from 0.5 to 6% by mass. The solid(rubber) concentration in the added natural rubber latex is preferablyfrom 0.2 to 5% by mass, more preferably from 0.25 to 1.5% by mass. Inthese cases, a rubber wet master batch can be produced which isheightened in carbon black dispersibility therein while the naturalrubber latex particles are certainly caused to adhere onto the carbonblack.

The method in the step (I) for mixing the carbon black with thedispersing solvent in the presence of the natural rubber latex may be amethod of using an ordinary disperser, such as a high-shearing mixer, aHigh Shear Mixer, a homo-mixer, a ball mill, a bead mill, ahigh-pressure homogenizer, an ultrasonic homogenizer or a colloid mill,to disperse the carbon black.

The “high-shearing mixer” denotes a mixer having a rotor rotatable at ahigh velocity and a fixed stator in which the rotor is rotated in thestate that a precise clearance is set between the rotor and the stator,whereby a high-shearing effect acts. In order to generate such ahigh-shearing effect, it is preferred to set the clearance between therotor and the stator, and the peripheral velocity of the rotor to 0.8 mmor less, and 5 m/s or more, respectively. Such a high-shearing mixer maybe a commercially available product. An example thereof is a product“High Shear Mixer” manufactured by Silverson.

In the case of mixing the carbon black and the dispersing solvent witheach other in the presence of the natural rubber latex in the presentinvention to produce the slurry solution containing thenatural-rubber-latex-particle-adhering carbon black, a surfactant may beadded thereto to improve the carbon black in dispersibility. Thesurfactant may be a surfactant known in the rubber industry. Examplesthereof include nonionic surfactants, anionic surfactants, cationicsurfactants, and amphoteric surfactants. Instead of the surfactant or inaddition to the surfactant, an alcohol such as ethanol may be used.However, it is feared that the use of the surfactant is to make rubberproperties of the final vulcanized rubber low. Thus, the blend amount ofthe surfactant is preferably 2 parts or less by mass, more preferably 1part or less by mass for 100 parts by mass of the solid (rubber) in thenatural rubber latex. It is preferred not to use any surfactantsubstantially. In order to restrain a deterioration of the solid(rubber) in the natural rubber latex in the steps (I) and (II), ananti-aging agent may be added thereto. The anti-aging agent may be ananti-aging agent known in the rubber industry. Examples thereof includeamine type, phenol type, organic phosphite type, and thioether typeagents.

About the natural-rubber-latex-particle-adhering carbon black in theslurry solution produced in the step (I), the 90% volume particle size(μm) (“D90”) is preferably 31 μm or more, more preferably 35 μm or more.In this case, the carbon black is excellent in dispersibility in theslurry solution and can be further prevented from being re-aggregated,so that the slurry solution is excellent in storage stability andfurther the final vulcanized rubber is also excellent in low-thermogenicperformance, endurance and rubber strength. In the present invention,the D90 of the natural-rubber-latex-particle-adhering carbon black meansa value obtained by making a measurement about the carbon black plus theadhering natural rubber latex particles.

(2) Step (II)

The step (II) is a step of mixing the slurry solution with the rest ofthe natural rubber latex to produce a rubber latex solution containingthe natural-rubber-latex-particle-adhering carbon black. The method formixing the slurry solution with the rest of the natural rubber latex ina liquid phase is not particularly limited, and may be a method of usingan ordinary disperser, such as a high-shearing mixer, a High ShearMixer, a homo-mixer, a ball mill, a bead mill, a high-pressurehomogenizer, an ultrasonic homogenizer or a colloid mill, to mix theslurry solution with the rest of the natural rubber latex solution. Atthe time of the mixing, the whole of the mixing system, such as thedisperser, may be optionally heated.

In the rest of the natural rubber latex, the solid (rubber)concentration is preferably higher than in the natural rubber latexadded in the step (I) when the drying period and labor thereof in thenext step (III) are considered. Specifically, the solid (rubber)concentration is preferably from 10 to 60% by mass, more preferably from20 to 30% by mass.

(3) Step (III)

The step (III) is a step of solidifying the rubber latex solutioncontaining the natural-rubber-latex-particle-adhering carbon black toproduce a carbon-black-containing rubber solidified product. The methodfor the solidification is, for example, a method of incorporating asolidifier into the rubber latex solution containing thenatural-rubber-latex-particle-adhering carbon black to solidify thelatex solution.

The solidifier used in the solidifying step may be an acid, such asformic acid or sulfuric acid, or a salt, such as sodium chloride, thatis usable usually for solidifying a rubber latex solution.

In the natural rubber wet master batch obtained through step (III), itis preferred about the ratio between the rubber components and thecarbon black that the filler is contained in an amount of 40 to 70 partsby mass for 100 parts by mass of the rubber (solid). This case makes itpossible to produce a natural rubber wet master batch which is improved,with a good balance, in carbon black dispersibility therein, and inlow-thermogenic performance and endurance to be attained when the masterbatch is finally made into a vulcanized rubber.

The rubber composition according to the present invention is acomposition obtained by dry-mixing the above-mentioned natural rubberwet master batch with a dry rubber made mainly of a polybutadienerubber, and an oil.

The species of the polybutadiene rubber (BR) may be a speciessynthesized, using a cobalt (Co) catalyst, neodymium (Nd) catalyst,nickel (Ni) catalyst, titanium (Ti) catalyst, or lithium (Li) catalyst;or a species synthesized, using a polymerization catalyst compositioncontaining a metallocene complex described in WO 2007/129670. In orderto improve the rubber composition in abrasion resistance, workability,tearing resistance and low-thermogenic performance with a good balance,it is preferred to blend thereinto a polybutadiene rubber having amass-average molecular weight of 350,000 to 1,000,000. It isparticularly preferred to blend thereinto a polybutadiene rubber havinga mass-average molecular weight of 350,000 to 1,000,000 and acis-1,4-isomer content of 95% or more. When the total amount of therubber composition is regarded as 100 parts by mass, the blend amount ofthe polybutadiene rubber is set preferably into the range of 20 to 50parts by mass.

As far as the rubber composition in the present invention contains atleast not only 50 parts or more by mass of the natural rubber but also20 to 50 parts by mass of the polybutadiene rubber in 100 parts by massof the rubber components, a rubber different from any polybutadienerubber may be blended as another species of the dry rubber. Examples ofthe blendable different rubber include polyisoprene rubber (IR),polystyrenebutadiene rubber (SBR), chloroprene rubber (CR), and nitrilerubber (NBR). These may be used alone or in the form of a blend of twoor more thereof.

The oil used in the present invention is an oil having a pour point of−10° C. or lower, and an aniline point of 90° C. or higher. By blending,in the rubber composition, this oil, the natural rubber wet masterbatch, and the polybutadiene rubber at a ratio in the specific range,the rubber composition is largely improved in braking performance on iceroad surfaces and low-thermogenic performance when used as a rawmaterial for tires, for example, their treads. As this oil, acommercially available product is also suitably usable. Examples thereofinclude a product “PROCESS P200” (pour point: −15° C., aniline point:102.3° C.) manufactured by JOMO, and a product “PS-32” (pour point: −20°C., aniline point: 110° C.) manufactured by Idemitsu Kosan Co., Ltd. Theblend amount of the oil in the rubber composition is preferably from 15to 40 parts by mass.

The rubber composition according to the present invention can beproduced by, for example, a rubber composition producing methodincluding step (I) in which when a carbon black is dispersed into adispersing solvent, at least one portion of a natural rubber latex isadded thereto, thereby producing a slurry solution containing the carbonblack to which natural rubber latex particles adhere, step (II) ofmixing this slurry solution with the rest of the natural rubber latexsolution to produce a natural rubber latex solution containing thejust-above described natural-rubber-latex-particle-adhering carbonblack, step (III) of solidifying the rubber latex solution containingthe natural-rubber-latex-particle-adhering carbon black to produce acarbon-black-containing natural rubber solidified product, and thendrying the solidified product to produce a natural rubber wet masterbatch, and step (IV) of dry-mixing this natural rubber wet master batchwith a dry rubber made mainly of a polybutadiene rubber, and an oil,wherein when the total amount of rubber components in the rubbercomposition is regarded as 100 parts by mass, the natural rubber iscontained in an amount of 50 parts or more by mass, and thepolybutadiene rubber is contained in an amount of 20 to 50 parts bymass, and the oil has a pour point of −10° C. or lower, and an anilinepoint of 90° C. or higher, and the blend amount of the oil is from 15 to40 parts by mass for 100 parts by mass of the rubber components.

It is particularly preferred that the step (III) has at least thefollowing dehydrating step (III-A) and drying plasticizing step (III-B)and no cooling step is set between the dehydrating step (III-A) and thedrying plasticizing step (III-B): the dehydrating step (III-A) of usinga first uniaxial extruder to dehydrate the carbon-black-containingnatural rubber solidified product while the product is heated up to 100to 180° C., thereby producing a natural rubber wet master batch; and thedrying plasticizing step (III-B) of using a second uniaxial extruder toplasticize the natural rubber wet master batch while the master batch isheated up to 120 to 180° C., thereby producing the natural rubber wetmaster batch further decreased in water content by percentage. This casemakes an improvement of a vulcanized rubber from the resultant rubbercomposition in rubber physical properties such as tearing resistance andhigh-strain-region stress property. Hereinafter, the step (III) will bedetailed.

Dehydrating Step (III-A):

The dehydrating step (III-A) is a step of using a first uniaxialextruder to dehydrate the carbon-black-containing natural rubbersolidified product while the product is heated up to 100 to 180° C.,thereby producing a natural rubber wet master batch. The first uniaxialextruder may be any uniaxial extruder usable in an ordinary rubberindustry. The barrel diameter (D), the barrel length (L), and furtherthe ratio of the barrel length to the barrel diameter (L/D) may be setat will. The gap width (slit width) between the inner wall of the barreland its screw is preferably from 0.1 to 0.9 mm. In the presentinvention, it is preferred to use a uniaxial extruder having no pinportions projected inward from the barrel inner wall of a dischargingport side region (expander region) of the uniaxial extruder. If theexpander region has pin portions, a high shearing force acts onto therubber components passing by the pin portions so that polymer chains inthe rubber components are cleaved. Thus, deterioration of the rubbercomponents advances easily. As a result, the vulcanized rubber to befinally obtained tends to be deteriorated in tearing resistance andhigh-strain-region stress property.

In the dehydrating step (III-A), the set temperature of the inside ofthe barrel of the first uniaxial extruder (the heating temperature forthe filler-containing rubber solidified product) is preferably from 160to 220° C., more preferably from 180 to 200° C. to decrease the watercontent by percentage efficiently in the resultant rubber wet masterbatch while the heat capacity and mechanical energy applied thereto arerestrained as much as possible.

In the natural rubber wet master batch obtained through the dehydratingstep, the water content by percentage is preferably from 1 to 10%, morepreferably from 1 to 8%.

Drying Plasticizing Step (III-B):

The drying plasticizing step (III-B) is a step of using a seconduniaxial extruder to plasticize the natural rubber wet master batchwhile the master batch is heated up to 120 to 180° C., thereby producingthe natural rubber wet master batch further decreased in water contentby percentage. The second uniaxial extruder may be equivalent to thefirst uniaxial extruder. In the same manner as in the first uniaxialextruder, the second uniaxial extruder preferably has no pin portions inthe barrel inner wall of its expander region.

In the drying plasticizing step (III-B), the set temperature of theinside of the barrel of the second uniaxial extruder (the heatingtemperature for the natural rubber wet master batch) is preferably from160 to 220° C., more preferably from 160 to 200° C. to decrease thewater content by percentage efficiently in the resultant rubber wetmaster batch while the heat capacity and mechanical energy appliedthereto are restrained as much as possible.

In the drying plasticizing step (III-B), it is preferred that themechanical energy applied to the natural rubber wet master batch in thesecond uniaxial extruder is 70 W/kg or less. This case finally gives,from the natural rubber wet master batch as a raw material, a vulcanizedrubber excellent in tearing resistance and high-strain-region stressproperty.

In the natural rubber wet master batch obtained through the dryingplasticizing step (III-B), the water content by percentage is preferably0.9% or less.

When the natural rubber wet master batch is plasticized in the dryingplasticizing step (III-B), the addition and incorporation of ananti-aging agent thereinto make it possible to disperse the anti-agingagent more evenly in the natural rubber wet master batch. The finalvulcanized rubber can be favorably prevented, with a higher certainty,from being rubber-deteriorated. The anti-aging agent may be ananti-aging agent usable ordinarily for rubbers, and examples thereofinclude aromatic amine type anti-aging agents, amine-ketone typeanti-aging agents, monophenolic type anti-aging agents, bisphenolic typeanti-aging agents, polyphenolic type anti-aging agents, dithiocarbamicacid salt type anti-aging agents, and thiourea type anti-aging agents.These may be used alone or in the form of an appropriate mixture. Thecontent of the anti-aging agent(s) is preferably from 0.3 to 3 parts bymass, more preferably from 0.5 to 1.5 parts by mass for 100 parts bymass of the rubber components (solid) in the rubber wet master batch.

Since the producing method according to the present embodiment has nocooling step between the dehydrating step (III-A) and the dryingplasticizing step (III-B), the water content by percentage can beefficiently decreased in the resultant natural rubber wet master batchwhile the heat capacity and mechanical energy applied to the masterbatch are restrained as much as possible. The method for preventing thenatural rubber wet master batch from being cooled between thedehydrating step (III-A) and the drying plasticizing step (III-B) maybe, for example, a method of connecting the first and second uniaxialextruders to each other through a connecting tool, such as a heatablecylinder having a short barrel length, or a method of connecting thefirst and second uniaxial extruders directly to each other. In order toprevent the natural rubber wet master batch from being cooled, thefollowing temperature is set preferably to 40° C. or higher, morepreferably to 60° C. or higher, in particular preferably to 120° C. orhigher: the temperature of the natural rubber wet master batch beforethe master batch is charged from the first uniaxial extruder into theconnecting tool; or the temperature of the natural rubber wet masterbatch before the master batch is charged into the second uniaxialextruder when the first and second uniaxial extruders are connecteddirectly to each other.

Shaping Plasticizing Step (III-C):

In the rubber composition producing method according to the presentembodiment, the step (III) may have, after the drying plasticizing step(III-B), a shaping plasticizing step (III-C) of using a mixer further toplasticize the above-mentioned natural rubber wet master batch. Themixer is preferably, for example, an open roll or a uniaxial extruder.In the shaping plasticizing step (III-C) also, it is preferred that themechanical energy applied to the natural rubber wet master batch in themixer is 70 W/kg or less since this case is to make the vulcanizedrubber to be finally obtained from the natural rubber wet master batchas a raw material excellent in tearing resistance and high-strain-regionstress property. The shaping machine may be a baler. In the dryingplasticizing step (III-B), the water content by percentage in thenatural rubber wet master batch has been sufficiently decreased;accordingly, the water content by percentage in the natural rubber wetmaster batch obtained through the shaping plasticizing step (III-C) maybe about 0.9% or less as in the case with the natural rubber wet masterbatch after the drying plasticizing step (III-B).

Dry-Mixing Step (IV):

In the dry-mixing step (IV), the natural rubber wet master batch isdry-mixed with a dry rubber made mainly of a polybutadiene rubber, andan oil. As a dry rubber other than the polybutadiene rubber, the same asdescribed above as the different rubber may be optionally blended. Inthe dry-mixing step (IV), at least one of a plant granular material, agrain granular material, and a granular region of a grain core materialmay be blended into the rubber composition.

The plant granular material is a granular material obtained bypulverizing shells of seeds of a walnut, a camellia or the like, ornuclei of fruits of a peach, a Japanese apricot or the like by a knownmethod, these shells or nuclei having a hardness larger than ices, i.e.,a Mohs' hardness of 2 or more. The plant granular material is projectedfrom the rubber surface to produce a road-surface scratching effect,thereby exhibiting an effect of preventing the rubber from slipping onice road surfaces. In order for the plant granular material to ensurebondability onto the rubber, the material is preferably a plant granularmaterial subjected to surface treatment for improving therubber-bondability thereof. When the material drops away from the phaseof the rubber, fine voids are generated to produce a water absorbingeffect. When at least one of the plant granular material, the graingranular material, and the granular region of the grain core material isblended, the blend amount thereof is preferably from 0.5 to 10 parts bymass for 100 parts by mass of the rubber components in the rubbercomposition. Specific examples thereof include a pulverized material ofnuts or shells of fruits such as a peach, Japanese apricot, walnut,ginkgo nut, peanut, or Japanese chestnut; grains such as rice, barley,wheat, foxtail millet, Japanese millet and corn; and core materialsthereof.

The dry-mixing step (IV) further has at least a kneading step (IV-A) anda vulcanization-related blending agent kneading step (IV-B).

Kneading Step (IV-A):

The kneading step (IV-A) is a step of charging the polybutadiene rubber,the oil, and one or more optional blending agents other than anyvulcanization-related blending agent into the natural rubber wet masterbatch obtained through the drying plasticizing step (III-B) or theshaping plasticizing step (III-C), and then using a mixing/dispersingdevice to knead all the components. Examples of the blending agent(s)include another rubber, stearic acid, zinc flower, an anti-aging agent,silica, a silane coupling agent, a wax, and a working aid. When theblending agent(s) is/are mixed with the rubber components in thekneading step (IV-A), for example, the following advantages areproduced: a rubber product after the master batch is vulcanized is to beheightened in strength; the rubber is made good in rubber-kneadingworkability; and the rubber is prevented from being deteriorated byradicals generated by the cleavage of the rubber molecular chains. Inthe kneading step (IV-A), for example, a gear-engaging type Banburymixer, a tangential line type Banbury mixer, or a kneader is usable. Inparticular, the use of a gear-engaging type Banbury mixer is preferred.

Vulcanization-Related Blending Agent Kneading Step (IV-B):

One or more vulcanization-related blending agents, such as a vulcanizer,for example, sulfur, and/or a vulcanization promoter, are charged intothe rubber composition obtained through the kneading step (IV-A), andthen the entire components are kneaded and mixed with each other. Whenthe rubber composition obtained through the vulcanization-relatedblending agent kneading step (IV-B) is heated to a predeterminedtemperature or higher, the vulcanizer in the rubber composition reactswith the rubber molecules so that crosslinkage structures are formedbetween the rubber molecules. Thus, the molecules are made into athree-dimensional network to give rubber elasticity to the rubbercomposition.

It is sufficient that the sulfur is a sulfur species for ordinaryrubbers. Examples thereof include powdery sulfur, precipitated sulfur,insoluble sulfur, and highly dispersed sulfur. The sulfur content in therubber composition according to the present invention is preferably from0.3 to 6 parts by mass for 100 parts by mass of the rubber components.If the sulfur content is less than 0.3 parts by mass, the vulcanizedrubber is short in crosslinkage density to be lowered in rubber strengthand others. If the sulfur content is more than 6.5 parts by mass, thevulcanized rubber is deteriorated, particularly, in both of heatresistance and endurance. In order for the vulcanized rubber to ensurerubber strength satisfactorily and be further improved in heatresistance and endurance, the sulfur content more preferably ranges from1.5 to 5.5 parts by mass for 100 parts by mass of the rubber components.

The vulcanization promoter may be a vulcanization promoter commonly usedfor rubber vulcanization. Examples thereof include sulfenamide typevulcanization promoters, thiuram type vulcanization promoters, thiazoletype vulcanization promoters, thiourea type vulcanization promoters,guanidine type vulcanization promoters, and dithiocarbamic acid salttype vulcanization promoters. These may be used alone or in the form ofan appropriate mixture. The content of the vulcanization promoter(s) ispreferably from 1 to 5 parts by mass, more preferably from 1.5 to 4parts by mass for 100 parts by mass of the rubber components.

EXAMPLES

Hereinafter, this invention will be more specifically described bydemonstrating examples thereof. Raw materials and devices used thereinare as follows:

Used Raw Materials:

a) Carbon blacks:

Carbon black “N339”: “SEAST KH”; N₂SA: 91 m²/g (manufactured by TokaiCarbon Co., Ltd.),

Carbon black “N234”: “SEAST 7HM”; N₂SA: 119 m²/g (manufactured by TokaiCarbon Co., Ltd.), and

Carbon black “N550”: “SEAST SO”; N₂SA: 40 m²/g (manufactured by TokaiCarbon Co., Ltd.);

b) Dispersing solvent: Water;c) Natural rubber latex:

Natural rubber concentrated latex solution, manufactured by Regitex Co.,Ltd. (DRC (dry rubber content)=60%), mass-average molecular weight(Mw)=236,000;

d) Solidifier: Formic acid (adjusted into a pH of 1.2 by diluting a 10%solution of a first class 85%-concentration agent) (manufactured byNacalai Tesque, Inc.);e) Natural rubber: RSS #3;f) Polybutadiene rubber: “HIGHCIS BR” (manufactured by JSR Corporation;g) Silica: “NIPSIL AQ” (manufactured by Nippon Silica Industrial Co.,Ltd.);h) Silane coupling agent: “Si75” (manufactured by Degussa);

i) Oils:

Oil A: “PROCESS NC140”; pour point: 7.5° C., and aniline point: 91.2° C.(manufactured by JOMO),

Oil B: “PROCESS P200”; pour point: −15° C., and aniline point: 102.3° C.(manufactured by JOMO), and

Oil C: “PS-32”; pour point: −20° C., and aniline point: 110° C.(manufactured by Idemitsu Kosan Co., Ltd.),

j) Stearic acid: “LUNAC S-20” (manufactured by Kao Corporation);k) Zinc flower:

“Zinc flower class-1” (manufactured by Mitsui Mining & Smelting Co.,Ltd.);

l) Anti-aging agent: “ANTIGEN 6C” (manufactured by Sumitomo ChemicalCo., Ltd.);m) Wax: “OZOACE 0355” (manufactured by Nippon Seiro Co., Ltd.);n) Vulcanization promoter: “SOXINOL CZ” (manufactured by SumitomoChemical Co., Ltd.);o) Sulfur: “Powdery sulfur” (manufactured by Tsurumi Chemical IndustryCo., Ltd.);p) Plant granular material: “SOFT GRIT #46” (manufactured by NipponWalnut Co., Ltd.) treated with an ordinary RF adhesive; andq) Granular material of a grain core material: CORN COBS GRIT 40/60(corncob) manufactured by Nippon Walnut Co., Ltd.

Example 1

Natural rubber wet master batches were each produced by the followingmethod:

Solidifying Step:

Carbon black (N339) was added into a diluted natural rubber latex, theconcentration therein being adjusted to 0.5% by mass, so as to give acarbon black concentration of 5% by mass. A device, ROBOMIX,manufactured by Primix Corp. was used to disperse the carbon blacktherein (ROBOMIX conditions: rotation at 9000 rpm for 30 minutes) toproduce a slurry solution containing the carbon black to which naturalrubber latex particles adhered (step (I)).

To the slurry solution produced in the step (I), which contained thenatural-rubber-latex-particle-adhering carbon black, was added the restof the natural rubber latex solution (the solid (rubber) concentrationtherein was adjusted to 25% by mass by the addition of water) to adjustthe total of the solid (rubber) content therein and that in the naturalrubber latex solution used in the step (I) to 50 parts by mass. Next, amixer for household use, model SM-L56, manufactured by Sanyo ElectricCo., Ltd. was used to mix these components with each other (mixerconditions: rotation at 11300 rpm for 30 minutes) to produce acarbon-black-containing natural rubber latex solution (step (II)). Inthe carbon-black-containing natural rubber latex solution, 25 parts bymass of the carbon black was contained for 50 parts by mass of therubber components (solid).

A 10%-by-mass solution of formic acid in water as a solidifier was addedto the carbon-black-containing natural rubber latex solution produced inthe step (II) until the pH of the whole turned to 4 (step (III)). Ascreen (φ2 punching, manufactured by Toyo Screen Kogyo Co., Ltd.) wasused to remove water from the solution containing the resultantcarbon-black-containing natural rubber solidified product, therebyproducing a carbon-black-containing rubber solidified product having awater content of 65.1%. In order to further decrease the water contentby percentage, the solidified product may be centrifuged. An instrument,model H-22 (BS-030) manufactured by Kokusan Co., Ltd., may be used tosubject the solidified product to solid-liquid separation (separatingconditions: rotation at 29000 rpm for 10 minutes), thereby producing afiller-containing rubber solidified product having a water content of46.2%.

Dehydrating Step (III-A) and Drying Plasticizing Step (III-B):

A first uniaxial extruder (product number: model V-02, manufactured bySuehiro EPM Corporation; barrel diameter: 90 mm; “barrel length”/“barreldiameter” (L/D)=8.6; and slit widths between the barrel and the screw:0.7 mm, 0.5 mm, and 0.2 mm) was connected directly to a second uniaxialextruder (a uniaxial extruder identical with the first uniaxialextruder). The natural rubber wet master batch was subjected to theabove-defined dehydrating step and drying plasticizing step while thefollowing were each set into a value described in Table 1: the heatingtemperature, and the mechanical energy applied to the natural rubber wetmaster batch (WMB) in each of the steps; and the water content bypercentage in the natural rubber wet master batch obtained after each ofthe steps. As shown in Table 1, the temperature change between thedehydrating step (III-A) and the drying plasticizing step (III-B) wasonly 30° C. Thus, it is understood that the present process had nocooling step between the dehydrating step (III-A) and the dryingplasticizing step (III-B).

Dry-Mixing Step (IV):

Into a B-type Banbury mixer (manufactured by Kobe Steel, Ltd.) werecharged each of the natural rubber wet master batches obtained throughthe drying plasticizing step (III-B), a polybutadiene rubber, an oil,and various blending agents shown in Table 1, and then these componentswere mixed with each other to produce a rubber composition. This rubbercomposition was vulcanized at 150° C. for 30 minutes to produce avulcanized rubber.

Rubber Hardness of Vulcanized Rubber:

The rubber hardnesses of the vulcanized rubber were measured at 23° C.and −5° C., respectively, in accordance with JIS K6253.

Low-Temperature Performance Index:

A viscoelasticity meter manufactured by Toyo Seiki Seisaku-sho, Ltd. wasused to measure the storage elastic moduli E′ of the rubber compositionat respective temperatures of −5° C. and −25° C. and at a frequency of10 Hz, a static strain of 10%, and a dynamic strain of ±0.25%. Thereciprocal number of each of the resultant values was shown as an indexunder the condition that the value of Comparative Example 1 was regardedas 100. As the index is larger, the composition is smaller in storageelastic modulus E′ so that the composition product is wider in contactarea at low temperatures to be better in low-temperature performance.

Braking Performance on Ice Road Surface (Ice Braking Performance):

The 2000-cc FF car was run at a speed of 40 km/h on an ice road surface(−3±3° C.), and then the braking distance thereof was measured (theaverage value under the condition that n was 10) when the car wassubjected to ABS operation. The resultant value was shown as an indexunder the condition that the value of Comparative Example 1 was regardedas 100. As the numeral value is larger, the rubber composition is betterin that the braking distance is shorter.

Low-Thermogenic Performance of Vulcanized Rubber:

In accordance with JIS K6265, the low-thermogenic performance of theproduced vulcanized rubber was evaluated on the basis of the losstangent tans thereof. The tans was measured at 50 Hz, 80° C. and adynamic strain of 2%, using a rheospectrometer, E4000 manufactured byUBM, and the measured value was converted to an index. The index was anindex under the condition that the value of Comparative Example 1 wasregarded as 100, and the index was used to make an evaluation. As thenumerical value is smaller, the vulcanized rubber is lower-thermogenicto be better.

Abrasion Resistance Performance:

In accordance with JIS K6264, the rubber composition was measured at aslip ratio of 30%, an applied load of 40 N and a dropping sand amount of20 g/minute, and then evaluated on the basis of the measured result. Theresultant value was shown as an index under the condition that the valueof Comparative Example 1 was regarded as 100. As the numeral value islarger, the rubber composition is better in abrasion resistance.

TABLE 1 Com- parative Com- Com- Com- Exam- parative parative parativeple 1 Example 2 Example 3 Example 4 Example 1 Example 2 Example 3Example 4 Example 5 Step (I) Carbon black Species — N339 N339 N339 N339N339 N550 N339 N339 Blend amount for 100 parts by — 50 50 50 50 50 70 5050 mass of rubber components Added Rubber content in NR — 5 5 5 5 5 5 55 rubber “Added rubber latex — 0.5% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5% 0.5%latex species concentration” (solid (rubber) concentration (% by mass))Step (II) Added Rubber content in NR — 95 95 95 95 95 95 95 95 rubber“Added rubber latex —  25%  25%  25%  25%  25%  25%  25%  25% latexspecies concentration” (solid (rubber) concentration (% by mass)) Rubberamount in rubber wet master batch — 100 100 100 100 100 100 100 100 (thenumber of parts by mass when the total amount of rubber components wasregarded as 100 parts by mass) Step (III-A) Used mixer — UniaxialUniaxial Uniaxial Uniaxial Uniaxial Uniaxial Uniaxial Uniaxial extruderextruder extruder extruder extruder extruder extruder extruder Heatingtemperature — 160° C. 160° C. 160° C. 160° C. 160° C. 160° C. 160° C.160° C. Water content (%) — 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00Mechanical energy (Wh/kg) applied to WMB — 93 93 93 93 93 93 93 93 Step(III-B) Used mixer — Uniaxial Uniaxial Uniaxial Uniaxial UniaxialUniaxial Uniaxial Uniaxial extruder extruder extruder extruder extruderextruder extruder extruder Heating temperature — 160° C. 160° C. 160° C.160° C. 160° C. 160° C. 160° C. 160° C. Water content (%) — 0.84 0.840.84 0.84 0.84 0.84 0.84 0.84 Mechanical energy (Wh/kg) applied to WMB —61 61 61 61 61 61 61 61 Step (III-C) Used mixer — Uniaxial UniaxialUniaxial Uniaxial Uniaxial Uniaxial Uniaxial Uniaxial extruder extruderextruder extruder extruder extruder extruder extruder Heatingtemperature — 160° C. 160° C. 160° C. 160° C. 160° C. 160° C. 160° C.160° C. Water content (%) — 0.83 0.83 0.83 0.83 0.83 0.83 0.83 0.83Mechanical energy (Wh/kg) applied to WMB — 63 63 63 63 63 63 63 63 Step(IV) NR 50 — — — — — — — — WMB — 75 75 75 75 75 85 75 75 BR 50 50 50 5050 50 50 50 50 Carbon black Species N339 — — — — — — — — Blend amountfor 100 parts by 25 — — — — — — — — mass of rubber components Silica 2525 25 25 25 25 25 25 25 Si75 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 Oil A20 20 — — — — — — — Oil B — — 10 50 20 — 20 35 20 Oil C — — — — — 20 — —— Stearic acid 2 2 2 2 2 2 2 2 2 Zinc flower 2 2 2 2 2 2 2 2 2Anti-aging agent 2 2 2 2 2 2 2 2 2 Wax 2 2 2 2 2 2 2 2 2 Plant granularmaterial 2 2 2 2 2 2 2 2 — Granular material of grain core material — —— — — — — — 2 Vulcanization promoter 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5Sulfur 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 2.1 Vulcanized rubber propertiesHardness (23° C.) 51 48 49 42 47 47 47 47 47 Hardness (−5° C.) 61 55 5845 51 50 50 51 51 Low-temperature performance E′ (−5° C.) 100 96 97 9295 95 96 95 95 indexes E′ (−25° C.) 100 107 87 147 140 145 140 145 140Ice braking performance index 100 100 92 117 125 130 125 130 125Low-thermogenic performance (tanδ) 100 92 87 110 89 89 90 90 89 Abrasionresistance 100 100 105 80 97 97 97 98 97

It is understood from the results in Table 1 that the vulcanized rubberfrom the rubber composition according to each of Examples 1 to 4 waslargely decreased in E′ in the range of low temperatures, and wasfurther improved in ice braking performance and low-thermogenicperformance. However, the vulcanized rubber from the rubber compositionaccording to each of Comparative Examples 2 and 3 was deteriorated inice braking performance and low-thermogenic performance, and thevulcanized rubber from the rubber composition of Comparative Example 4was deteriorated in low-thermogenic performance and abrasion resistance.

1. A rubber composition obtained by dry-mixing a natural rubber wetmaster batch yielded by mixing at least a natural rubber latex and acarbon-black-containing slurry solution with each other in a liquidphase and drying the resultant mixture, a dry rubber made mainly of apolybutadiene rubber, and an oil, wherein when the total amount ofrubber components in the rubber composition is regarded as 100 parts bymass, the natural rubber is contained in an amount of 50 parts or moreby mass, and the polybutadiene rubber is contained in an amount of 20 to50 parts by mass, and the oil has a pour point of −10 C or lower, and ananiline point of 90 C or higher, and the blend amount of the oil is from15 to 40 parts by mass for 100 parts by mass of the rubber components.2. The rubber composition according to claim 1, wherein the naturalrubber wet master batch is a master batch produced through the followingsteps: step (I) in which when a carbon black is dispersed into adispersing solvent to prepare the carbon-black-containing slurrysolution, at least one portion of the natural rubber latex is addedthereto, thereby producing a slurry solution containing the carbon blackto which natural rubber latex particles adhere, step (II) of mixing thisslurry solution with the rest of the natural rubber latex to produce anatural rubber latex solution containing the just-above describednatural-rubber-latex-particle-adhering carbon black, and step (III) ofsolidifying and drying the natural rubber latex solution containing thenatural-rubber-latex-particle-adhering carbon black.
 3. The rubbercomposition according to claim 1, wherein the natural rubber wet masterbatch contains the carbon black in an amount of 40 to 70 parts by massfor 100 parts by mass of the rubber components of the master batch. 4.The rubber composition according to according to claim 1, wherein thecarbon black contained in the natural rubber wet master batch has anitrogen adsorption specific surface area (N2SA) of 100 m2/g or less. 5.The rubber composition according to according to claim 1, wherein whenthe total amount of the rubber components in the rubber composition isregarded as 100 parts by mass, at least one of a plant granularmaterial, a grain granular material, and a granular region of a graincore material is further contained in an amount of 0.5 to 10 parts bymass.